The VirtualBox provider can bootstrap
to both .vdi and .vmdk images (raw images are also supported but do not
run in VirtualBox). It’s advisable to always use vmdk images for
interoperability (e.g.
OVF files
should support vdi files, but since they have no identifier URL not
even VirtualBox itself can import them).

VirtualBox Guest Additions can be installed automatically if the ISO is
provided in the manifest.
VirtualBox Additions iso can be installed from main Debian repo by running:
apt install virtualbox-guest-additions-iso